Portable electronic equipment, electric vehicles (EVs), hybrid electric vehicles (HEVs), and other equipment that are becoming increasingly popular are requiring stricter specifications for the nonaqueous electrolyte secondary batteries employed in such equipment year after year. In particular, such a battery is required to be compact and lightweight and to have high capacity and output, excellent cycling characteristics, and stable performance.
An example of a nonaqueous electrolyte secondary battery is a lithium ion battery including a wound electrode assembly. The wound electrode assembly includes a positive electrode plate having a positive electrode active material layer on each face of a positive electrode substrate of long aluminum foil or other materials, a negative electrode plate having a negative electrode active material layer on each face of a negative electrode substrate of long copper foil or other materials, and a long separator of microporous polyolefin. The positive and negative electrode plates and the separator interposed therebetween are wound to form a columnar or flat electrode assembly. The positive electrode plate and the negative electrode plate are then connected to a positive electrode tab and a negative electrode tab, respectively, and the whole is stored in an outer body having a particular shape.
Another example is a lithium ion battery including a stacked electrode assembly. The stacked electrode assembly includes a positive electrode plate having a positive electrode active material layer on each face of a positive electrode substrate of aluminum foil or other materials, a negative electrode plate having a negative electrode active material layer on each face of a negative electrode substrate of copper foil or other materials, and a separator of microporous polyolefin. A plurality of layers of the positive and negative electrode plates are stacked with the separator interposed therebetween to form a stacked electrode assembly. Positive electrode tabs connected to the layers of the positive electrode plate are connected in parallel to each other, and negative electrode tabs connected to the layers of the negative electrode plate are connected in parallel to each other. The whole is stored in an outer body having a particular shape.
In the nonaqueous electrolyte secondary battery represented by such a lithium ion battery, in order for the negative electrode active material to smoothly absorb lithium ions desorbed from the positive electrode active material during charging, the negative electrode active material layer is typically arranged to protrude from the end of the positive electrode active material layer with the separator interposed therebetween. Thus, a positive electrode substrate exposed portion, which is formed on the positive electrode plate, faces the end of the negative electrode plate with the separator interposed therebetween.
In a battery having such a structure in which the positive electrode substrate exposed portion faces the end of the negative electrode plate with the separator interposed therebetween, burrs from the end of the negative electrode plate, powder from the electrode plate, or other substances may pierce the separator to cause short circuit. Such a short circuit may lead to a large current flow, resulting in heat emission.
To solve such a problem, techniques have been disclosed in which a protective layer is formed in a region adjacent to the positive electrode active material layer in a positive electrode substrate exposed portion on which a positive electrode active material layer is not formed.
For example, Patent Document 1 discloses a nonaqueous secondary battery in which an insulating layer of an insulating resin is formed on each face of the base sections of protrusions exposing metals of positive and negative electrodes in a wound electrode assembly. Thus, even if non-protruding sections of the positive and negative electrodes are exposed due to winding misalignment of a separator in the axial direction, the insulating layers prevent a short circuit between the base sections of the protrusions of the positive and negative electrodes and also prevent a short circuit caused by burrs due to, for example, punching of an electrode plate.
In addition, Patent Document 2 discloses a nonaqueous secondary battery including an active material mixture layer placed on at least one face of a collector of metal foil and a positive electrode having a portion exposing metal. The positive electrode together with the metal exposed portion faces a negative electrode with a separator interposed therebetween. A protective layer having a lower electronic conductivity than that of the metal and made of a non-insulating material is formed on an area of the metal exposed portion that faces the negative electrode with the separator interposed therebetween. If a part of one electrode pierces the separator to come in contact with the other electrode, such a structure can gently discharge a battery to thus prevent the battery from generating excessive heat and also allows equipment to detect the abnormality of the battery through the fall in battery voltage.